ADE7758 Analog Devices, ADE7758 Datasheet - Page 29

• Manufacturer Part Number: ADE7758
• Description: Poly Phase Multifunction Energy Metering IC with Per Phase Information
• Manufacturer: Analog Devices

equivalent to one LSB of the current waveform sample. The
update rate of the current rms measurement is CLKIN/12.
CURRENT SIGNAL
With the specified full-scale analog input signal of 0.5 V, the
ADC produces an output code that is approximately ±2,642,412d
(see the Current Channel ADC section). The equivalent rms
values of a full-scale sinusoidal signal at 60 Hz is 1,914,753
(0x1D3781).
The accuracy of the current rms is typically 0.2% error
from the full-scale input down to 1/500 of the full-scale input.
Additionally, this measurement has a bandwidth of 14 kHz.
Current RMS Offset Compensation
The ADE7758 incorporates a current rms offset compensation
for each phase (AIRMSOS, BIRMSOS, and CIRMSOS). These
are 12-bit signed registers that can be used to remove offsets in
the current rms calculations. An offset may exist in the rms
calculation due to input noises that are integrated in the dc
component of I
the IRMS registers to be maintained at 0 when no current is
being consumed. One LSB of the current rms offset register is
equivalent to 16,384 (decimal) of the square of the current rms
register. Assuming that the maximum value from the current
rms calculation is 1,868,467d with full-scale ac inputs, one LSB
of the current rms offset represents 0.94% of the measurement
error at −60 dB down of full scale. For details on how to
calibrate the current rms measurement, see the Calibration
section.
where IRMS
Voltage Channel RMS Calculation
Figure 63 shows the details of the signal path for the rms
calculation on Phase A of the voltage channel. The voltage
channel rms value is processed from the waveform samples
after the low-pass filter LPF1. The output of the voltage channel
ADC can be scaled by ±50% by changing VRMSGAIN[11:0]
registers to perform an overall rms voltage calibration. The
VRMSGAIN registers scale the rms calculations as well as the
apparent energy calculation, since apparent power is the
product of the voltage and current rms values. The voltage rms
values are stored in unsigned 24-bit registers (AVRMS, BVRMS,
0xD7AE14
FROM HPF OR
0x2851EC
(IF ENABLED)
INTEGRATOR
IRMS
0x0
=
0
is the rms measurement without offset correction.
IRMS
Figure 62. Current RMS Signal Processing
2
(t). The offset calibration allows the contents of
0
X
2
2
+
SGN 2
16
,
LPF3
384
25
2
×
AIRMSOS[11:0]
24
+
IRMSOS
+
2
23
2
17
0x1D3781
2
16
0x00
2
AIRMS[24:0]
15
Rev. A | Page 29 of 68
and CVRMS). The 256 LSBs of the voltage rms register is
approximately equivalent to one LSB of a voltage waveform
sample. The update rate of the voltage rms measurement is
CLKIN/12.
With the specified full-scale ac analog input signal of 0.5 V, the
LPF1 produces an output code that is approximately 63% of its
full-scale value , i.e., ±9,372d, at 60 Hz (see the Voltage Channel
ADC section). The equivalent rms value of a full-scale ac signal
is approximately 1,639,101 (0x1902BD) in the VRMS register.
The accuracy of the VRMS measurement is typically 0.5% error
from the full-scale input down to 1/20 of the full-scale input.
Additionally, this measurement has a bandwidth of 160 Hz.
Voltage RMS Offset Compensation
The ADE7758 incorporates a voltage rms offset compensation
for each phase (AVRMSOS, BVRMSOS, and CVRMSOS).
These are 12-bit signed registers that can be used to remove
offsets in the voltage rms calculations. An offset may exist in the
rms calculation due to input noises and offsets in the input
samples. It should be noted that the offset calibration does not
allow the contents of the VRMS registers to be maintained at 0
when no voltage is applied. This is caused by noise in the
voltage rms calculation, which limits the usable range between
full scale
and 1/50th of full scale. One LSB of the voltage rms offset is
equivalent to 64 LSBs of the voltage rms register.
Assuming that the maximum value from the voltage rms
calculation is 1,639,101d with full-scale ac inputs, then 1 LSB
of the voltage rms offset represents 0.042% of the measurement
error at 1/10 of full scale.
where VRMS
correction.
VAN
VOLTAGE SIGNAL–V(t)
AVRMSGAIN[11:0]
VRMS
GAIN
0.5
=
0
VRMS
is the rms measurement without the offset
Figure 63. Voltage RMS Signal Processing
LPF1
0
+
50Hz
60Hz
VRMSOS
0xDA5E
0xDB64
0x25A2
0x249C
X
2
0x0
0x0
LPF3
LPF OUTPUT
WORD RANGE
LPF OUTPUT
WORD RANGE
×
64
SGN 2
16
2
VRMSOS[11:0]
15
50Hz
60Hz
2
0x1902BD
+
0x193504
14
+
ADE7758
0x0
0x0
AVRMS[23:0]
2
8
2
7
2
6